The abstract noun "strength" can be stated as an adjective. Alternate translation: "You make them wonderfully strong" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
Here "their" refers to the Israelites. The writer would have included himself as a member of the Israelites. Alternate translation: "You are our" (See: [[rc://en/ta/man/translate/figs-explicit]])